The present invention relates in general to a method of and apparatus for producing pipes, and more particularly to a method of an apparatus for producing plastic pipes by the extrusion process in which the thickness of the plastic pipe is controlled by the takeaway speed of the molten plastic pipe.
Plastic pipes have been produced by a method commonly referred to as an extrusion process. In the extrusion process, dry raw materials have been placed within a hopper for passage to an extruder. The extruder employed one or more screw type devices which had kneed and compressed the raw material. Heat was applied to the extruder and the combination of the heat and applied pressure turned the raw material into a molten plastic material. At the discharge end of the extruder, the molten plastic material was forced through a die, which formed the molten plastic material into a molten plastic hollow tube.
The molten plastic hollow tube had the approximate dimensions of the desired product. After the molten plastic tube was discharged from the extruder, it was advanced through a sizing sleeve, which was disposed in a cooling tank. The outside diameter of the plastic pipe to be formed therefrom was determined by the inside diameter of the sizing sleeve. As the molten plastic pipe advanced through the sizing sleeve, an external pressure was applied to the inner wall of the molten plastic pipe to urge the outer wall of the molten plastic pipe against the inner wall of the sizing sleeve. The length of the sizing sleeve, which was disposed in the cooling tank, was great enough to allow the skin, outer diameter surface, to cool and thus become rigid while being discharged from the sizing sleeve.
The entire plastic pipe was solidified or made rigid after it was advanced in the cooling tank beyond the sizing sleeve. In the cooling tank were spray heads or chilled water, which had extracted heat from the molten plastic pipe.
The inside diameter of the molten plastic pipe and, therefore, the thickness of the molten plastic pipe was controlled by the speed at which the molten plastic pipe was removed from the sizing sleeve. Thus, it was the takeaway speed at which the molten plastic pipe was removed from the sizing sleeve that had controlled the thickness of the rigid plastic pipe. The faster the rate of withdrawal of the molten plastic pipe from the sizing sleeve, the lesser the thickness of the plastic pipe, since the extruder operated a fixed extrusion rate and the outer diameter of the molten plastic pipe was set by the rigid outer skin thereof being constantly urged against the inner wall of the sizing sleeve.
After the rigid plastic pipe had advanced beyond the cooling tank, it was gripped by takeaway rollers. The speed at which the takeaway rollers were operated controlled the speed at which the molten plastic pipe was removed from the sizing sleeve. A d.c. drive motor controlled the speed at which the takeaway rollers operated. It was a d.c. motor controller, such as a speed control potentiometer driven by a stepping type motor, that was adjusted to regulate the speed at which the d.c. drive motor operated.
Heretofore, the thickness of the plastic pipes produced by the extrusion process depended on the accuracy of the extruder in the rate of feeding the molten plastic tubes to the sizing sleeve, the accurcy of the d.c. drive motor in maintaining a set speed for operating the takeaway rollers, and the frequency at which the thickness of the rigid plastic pipe was inspected.
In order to obtain proper pipe strength, the plastic pipes must meet specific minimum wall thickness. Therefore, manufacturers had a tendency to exceed the mininum wall thickness. This has been a rather costly practice for plastic pipe manufacturers.
Attempts have been made to measure the wall thickness of plastic pipes during the process of manufacturing the same in an effort to reduce manufacturing waste. One measuring device manufactured by Nucleonics used radiation scattering techniques. The equipment was mounted downstream of the cooling tank along the takeaway line. It gave a digital readout of the wall thickness of the plastic pipe. An operator manually adjusted the takeaway speed, based on the digital readout, to regulate the d.c. motor speed, and the speed of the takeaway rollers. Other equipment used for measuring the wall thickness of plastic pipes was manufactured by LFE, which was an ultrasonic unit. The ultrasonic unit was mounted in the cooling tank at the downstream end of the sizing sleeve and had a digital readout. The takeway speed of the plastic pipe was controlled by regulating the speed of the d.c. drive motor through a speed control potentiometer operated through a stepping motor.
In the ultrasonic equipment heretofore employed for measuring the wall thickness of plastic pipes, the ultrasonic transducer sent out pulses through the plastic pipe and detected the reflected pulses. The time interval variations between the transmitted pulses and the detected reflected pulses represented the thickness of the wall of the plastic pipe under test. Such ultrasonic equipment were installed downstream of the sizing sleeve. The transducers of such ultrasonic equipment were not disposed at the upstream end of the sizing sleeve because the plastic was still in a molten state and any engagement therewith resulted in undesirable markings on the finished product. Therefore, the transducers were placed downstream of the sizing sleeve to obviate the undesirale marking on the finished product and also to use the chilled water in the cooling tank to couple the sonic pulses to the plastic pipe to lessen the wear on the transducer resulting from the advancing plastic pipe.
The various systems heretofore employed for measuring and controlling the wall thickness of plastic pipes made by the extrusion process have not been totally satisfactory. Firstly, the adjustment of the takeaway speed was only as good as the ability of the d.c. drive motor to maintain a constant adjusted speed. Secondly, the taking of thickness measurements at the downstream end of the sizing sleeve or a distance downstream from the point of wall thickness generation in the sizing sleeve resulted in a time lag between the generation of the error and the detection and/or correction of the error.